This invention relates to microoptical systems and more particularly to optical waveguides.
Heretofore, optical waveguides have been formed of dielectric solid state materials wherein the optical wave is totally reflected along a path in which the optical medium has an index of refraction which is greater than the adjacent mediums. Such waveguides have been set forth in an article, "Light Waves In Thin Films and Integrated Optics" by P. K. Tien, Applied Optics, 10, page 2395, 1971. Passive optical microcircuits having thin-film lenses, prisms, etc. have been constructed and have demonstrated that passive optical processing functions can be performed in integrated optical circuits. Active optical devices have been proposed; however, these have been demonstrated only in a very limited number of material waveguide systems. These devices make use of solid state waveguides as well as a liquid medium. Prior art optical waveguides can be thought of as a slab of dielectric which confines light by multiple total internal reflections. Thus, consider three media (FIG. 1(a)); a thin film (n.sub.1), a substrate (n.sub.0), and an unspecified medium (n.sub.2). The plane wavefront is totally reflected alternately between the interfaces S.sub.1 and S.sub.2. For this to happen the index of refraction n.sub.1 of the guiding dielectric must be greater than the indices n.sub.0 and n.sub.2 of the surrounding media and the angle of incidence .theta. must be greater than the larger of the two critical angles .theta..sub.c0,2, where ##EQU1##
For a given thickness 2d and indices n.sub.0, n.sub.1, and n.sub.2 light will propagate with an angle of incidence .theta. only if, after two successive reflections, the wavefront is again in phase with any portion of the original wavefront that was not involved in these two reflections. If this were not the case, after many reflections wavefronts with a range of phases between 0 and 2.pi., would add to zero amplitude or, equivalently, the wave would not propagate. The requirement implies that at an arbitrary point X the phase of one wavefront obtained from another by two successive reflections must equal the phase of the other wavefront at X without reflections, or differ by a multiple of 2.pi.. This requirement permits propagation for only discrete values of .sigma.. Each value of .sigma. is associated with a mode of propagation and each mode has a characteristic velocity of propagation. As .sigma. increases, the velocity of propagation to the right also increases. One can associate a different effective index of refraction, which is a function of d, n.sub.1, n.sub.2 and n.sub.0, for each waveguide mode.
Now consider those properties of waveguides which lead to low-loss propagation, to techniques of input and output coupling, and to opticl and switching and modulation. There is considerable discussion in the literature concerning various topological shapes of optical waveguides and their characteristics. Waveguides may be flat slabs, rectangular, cylindrical, or any other shape. Metal-clad dielectrics may not be used in optical waveguides due to the high losses. When light is reflected off a metal surface, the energy losses depend on the metal, the wavelength of the light, surface condition, polarization, and angle of incidence, and the losses are typically about 1 or 2% per reflection. Hence, metal-clad waveguides, because of the number of reflections per centimeter, are extremely lossy in the optical region. In the dielectric-clad waveguide, losses are due to absorption in the dielectrics and scattering losses. If the cladding has no absorption at the optical wavelength being propagated, then no energy is absorbed from the evanescent wave (extending into the cladding) and the waveguide suffers only from scattering losses.
The boundary between the waveguide and the surrounding media must be clean, smooth, and free from scratches inside to minimize surface scatter. Deviations of the waveguide wall by a few percent can cause a power loss of 0.5 dB/cm if the wall imperfection can be described by an exponential correlation function. Besides the lack of a flat, smooth surface, inhomogeneities in the waveguide or cladding media can also lead to significant waveguide losses. This is particularly important in the case of liquid-crystals, since the molecular alignment is affected by the walls and the lack of perfect molecular alignment can lead to large losses; these losses could be the dominant waveguide loss mechanism.
One technique of a high degree of molecular alignment involves uniform rubbing of the liquid-crystal boundaries and is referred to as the homogeneous alignment technique. This presumably scratches the surface and provides a preferred direction in which the molecules align. The homogeneous alignment technique in principle may not be compatible with low-loss waveguide fabrication. However, it is found in practice that this fabrication technique indeed does lead to the fabrication of low-loss waveguides. Another alignment technique referred to as the homeotropic alignment technique, involves the deposition of a thin film of polar atoms (surfactant) on surface. Again a preferred alignment direction is established and a high degree alignment is possible, usually at right angles to the alignment achieved by rubbing. This method seems more applicable to waveguide techniques and requirements.
To couple light into and out of a waveguide, it is necessary to change the boundary conditions. This may be done in several ways. In the prism coupler a prism is brought within a few optical wavelengths of a waveguide surface and frustrates total internal reflection at the surface. Frustrated total internal reflection or evanescent-field coupling occurs because the boundary conditions of a waveguide are modified by the presence of the prism. A second type of coupler, the grating coupler consists of a periodic structure in contact with the waveguide boundary. This structure permits momentum matching between a guided optical wave and a wave propagating in the cladding medium and provides coupling between the two waves. Both types of couplers may be fabricated using liquid-crystals.
A third technique which has proved to be the most successful in liquid-crystal waveguides is to couple into another thin film, such as a polymer film first. This thin film is brought in contact with the liquid-crystal thin film and light is coupled from the polymer thin film into the liquid-crystal thin film. An example of the use of this type of coupler is given in the article "Electrooptical Switching in Low Loss Liquid-Crystal Waveguides" by J. P. Sheridan, J. Schnur and T. G. Giallorenzi; Applied Physics Letters 22 560 (1973).
To construct active devices, the index of the waveguide media, or the determining boundary conditions, must be actively controlled by some external parameter. Once this is acheived, the construction of systems that will act as modulators, deflectors, and switches are possible.
Modulation may be achieved by electrooptic, acoustooptic, and magnetooptic phenomena. The electrooptic modulators make use of induced birefringence to cause phase changes in the optical waveguide modes. For example, in a waveguide of electrooptic material, or in a waveguide of an electrooptic substrate, the propagation factors for TE and TM modes vary differently with an applied electric field. If a TE wave is passed through a modulator, a phase shift may be induced, which causes a TE wave to be coupled to an orthogonally polarized TM mode. Viewed through an appropriate analyzer, the output is an amplitude-modulated signal. Magnetooptic modulators make use of induced optical activity instead of induced birefringence and require current-modulated signals instead of voltage-modulated signals.
Deflectors can operate either via the electrooptic or acoustooptic effect. In the case of electrooptic deflectors not used in optical waveguides, these devices are usually digital deflectors. The digital devices use the principle that the electrooptic effect rotates the plane of polarization of linearly polarized light in a suitable medium, so that a polarizing prism such as a Rochon, Wollaston, or Thompson prism deflects the beam into one of two channels. The same effect is accomplished in optical waveguides using coupled waveguides. No known exact analog of the digital deflector has been demonstrated to date in optical waveguides.
The acoustooptic deflector has the advantage of continuous deflection by electronic tuning of the acoustic frequency. The acoustic wave produces periodic index and surface variations, which in turn deflect a portion of the optical radiation by Bragg diffraction. This process is particularly adaptable to a dielectric film, which forms a surface layer on a substrate, producing a waveguide. In this case the acoustic wave is a surface wave, and the optical radiation is also on the surface of the substrate. The interaction can therefore be achieved with much less acoustic power than in the bulk case, and efficiency is improved. It has been determined that such a device had a maximum observed deflection efficiency of 66% with an electrical input of 2.5 W, or an acoustic power of 0.18 W. Liquid crystals have all the properties that are needed for switching, deflecting, and modulating and are able to combine the advantages of the acoustooptic and electrooptic devices in one compact package.